The present invention relates to an aqueous surface treatment chemical for forming hydrophilic coatings on heat exchanger fins, etc., a bath containing such a surface treatment chemical, a method of surface-treating an aluminum member with such an aqueous surface treatment chemical, and an aluminum member provided with a hydrophilic coating of such a surface treatment chemical. More particularly, it relates to an aqueous surface treatment chemical for forming hydrophilic coatings with good corrosion resistance and highly durable hydrophilic nature and also with no or extremely reduced odor on aluminum fins of heat exchangers, a bath containing such a surface treatment chemical and a method of treating them.
Aluminum and its alloys are light and have good workability and heat conductance, so that they are widely used for fins of heat exchangers. Recently more and more air conditioners have been used not only for cooling but also for warming and dehumidification. In heat exchange parts of these air conditioners, aluminum alloy fins are generally used.
However, it is observed that moisture tends to be condensed and deposited as water droplets on the fin surfaces of air conditioners during cooling operations. If the fin surface is water-repellent, this condensed water tends to be deposited in a hemispherical form on the fin surface or forms bridges between the fins, preventing smooth air flow, which in turn increases resistance of air flow, thereby decreasing heat exchange efficiency.
In addition, although aluminum and its alloys are essentially excellent in corrosion resistance, it is likely that the condensed water remaining on the aluminum fin surfaces for a long period of time functions like an oxygen concentration cell, and that contaminants such as nitrogen oxides NO.sub.x and sulfur oxides in the air are absorbed and concentrated in the condensed water. As a result, a hydration reaction and a corrosion reaction are accelerated. Those produced by the corrosion are accumulated on the aluminum fin surfaces, which not only deteriorate heat exchange performance, but also are blown out of the air conditioners as white fine powders together with a hot air during the warming operation in the winter.
To solve these problems, various attempts have been made to form on aluminum fin surfaces coatings for improving their corrosion resistance and for increasing their hydrophilic nature.
Such coatings are generally classified into inorganic coatings and organic coatings.
Proposed as methods for forming inorganic coatings are methods comprising forming a corrosion-resistant coating, for instance, by a chromate treatment, and dipping it in an aqueous solution mainly composed of boehmite (Japanese Patent Laid-Open No. 56-108071), or silicate to form a hydrophilic coating (Japanese Patent Laid-Open Nos. 56-13078 and 50-38645). However, despite the fact that such inorganic coatings have good corrosion resistance, they are poor in hydrophilic nature. Besides, in the case of a silicate treatment, it generates unpleasant odor peculiar to silica. In addition, when it is used to produce pre-coated coils, the coating is cracked while cutting so that its corrosion resistance is reduced, and also severe tool wear takes place by cutting operation of such precoated aluminum plates.
On the other hand, as a method of forming organic coating, a method of forming a hydrophilic coating on an aluminum surface by applying an aqueous solution comprising a water-soluble or water-dispersible, high-molecular weight resin and silica fine powders to the aluminum surface, and then heat-curing the coating was proposed (for instance, Japanese Patent Laid-Open Nos. 55-99976, 53-125437 and 55-164264). However, since this coating contains fine silica powder, it suffers from cracking while cutting in the case of a pre-coated aluminum plate, leading to lower corrosion resistance. And tools are extremely worn during cutting operations and the coating has relatively poor hydrophilic nature. In addition, it causes air conditioners to release unacceptable if slight odor peculiar to silica. Incidentally, Japanese Patent Laid-Open No. 55-164264 discloses the use of water soluble melamines, alkyds, polyesters, acrylics for the same purpose, but their properties are not sufficient.
Further, U.S. Pat. No. 4,503,907 discloses a heat exchanger coated with an aqueous coating composition comprising a plurality of spaced fins with narrow distance in parallel to form flow passages between fins and a plurality of heat transfer pipes passing through the fins, the both surfaces of the fins being coated with an aqueous coating composition comprising 100 parts by weight of a resin component for water paint in solid content, 5 to 95 parts by weight of a surfactant and 5 to 65 parts by weight of synthetic silica and baked at a temperature of 120.degree. C. to 200.degree. C. for 10 to 40 minutes for curing to give a coating film of 3 to 20 .mu.m, whereby the fins are provided with hydrophilic surfaces having excellent corrosion resistance and surface hardness. However, this coating composition also contains silica which causes the generation of unacceptable odor.
U.S. Pat. No. 4,588,025 discloses a heat exchanger made of aluminum and comprising a tube and fins attached to the tube, the heat exchanger being characterized in that the fins are treated with a coating composition comprising an alkali silicate, a low-molecular weight, organic compound having carbonyl group, and a water-soluble, high-molecular weight organic compound, and are thereafter dried by heating, whereby a hydrophilic coating is formed over the surfaces of the fins. This hydrophilic coating too releases unacceptable odor because of the silicate.
Further, Japanese Patent Laid-Open No. 61-101798 discloses a heat exchanger fin constituted by a thin aluminum plate, which is coated with a first layer made of a water-soluble acrylic resin, and a second layer made of a water-soluble cellulose resin. However, it shows poor water dissolution resistance.
In addition, Japanese Patent Laid-Open No. 62-105629 discloses an aluminum heat exchanger fin coated with a corrosion-resistant layer comprising an organic layer made of a water-soluble acrylic resin or a water-soluble urethane resin or an inorganic layer made of a chromate layer, a boehmite layer or an anodized layer, and a hydrophilic layer formed thereon, which comprises one or more organic compounds selected from water-soluble cellulose resins and polyvinyl alcohol and a curing agent such as melamine resins. However, this hydrophilic coating is also insufficient in hydrophilic nature and water dissolution resistance.
As described above, the conventional surface treatments of heat exchanger fins fail to provide surface coatings having sufficient hydrophilic nature and corrosion resistance and high mechanical strength without generating unacceptable odor.